Typically in an aircraft, aircraft cabin air pressure in terms of pressure altitude is in the range of 3000 to 8000 feet, which is generally less than a pressure encountered at a ground level. Persons with impaired pulmonary capacity are not fit to travel in the reduced aircraft cabin air pressure associated with low oxygen levels (e.g., due to recirculation of aircraft cabin air by air conditioning/environmental control system (ECS) in the aircraft). This is especially true for persons suffering or predisposed to conditions including but not limited to chronic bronchitis, emphysema, bronchiectasis, dyspnoea at rest, corpulmonale, severe asthma, anemia (sickle cell hemoglobin and betathalassaemia) and the like. This can also include persons who have undergone recent lung, chest injury/surgery/pulmonary infections. That is, the persons to whom exposure to higher altitudes/low oxygen levels normally encountered in an aircraft cabin may cause under oxygenation of blood hemoglobin and subsequent tissue hypoxia.
Currently, such individuals are transported using a flight that provides special oxygen supply and cabin altitude not exceeding a guaranteed 3500/4000 feet ambient. This may require flying at an extraordinarily uneconomical altitude for the aircraft or evacuating using dedicated military aircraft (such as turboprop or chartered flights) with large volume oxygen supply on board. In either case, it is a high cost that is generally not covered by social health schemes and health insurances. For short distances, helicopters are used typically for such purposes.
However, none of these current solutions are economically viable as they all require flying at nearly surface level, monitoring and adjusting oxygen by medical attendants, remaining on a large volume oxygen supply, and so on. Further, today's demand dilution oxygen regulators for aviation use operate above a pressure altitude of 10000 to 12000 feet.